The present invention relates to an electric generator, in particular for motor vehicles in which the internal combustion engine drives the generator and the latter charges a current-accumulator/battery. Modern motor vehicles have a large number of current-consuming devices such as lights, wiper motors, radio/recorders, fuel pumps, water pumps for windshield-wiping and washing systems, electric fans, heating fans, rear-window heating, power windows, etc. The electric power consumption of the electrical system of a motor vehicle is today already between 800 and 1100 watts, without taking into consideration the short-time current-consuming device, namely the starter, which, depending on its arrangement, may consume 800 to 3000 watts.
The generators must be so designed that upon normal operation of the vehicle on highways, super highways and in city traffic, the average generator current exceeds the sum of the current of all consumption devices so that the battery is charged. The generator current is not available with speeds of rotation of the generator of up to 1000 rpm and it reaches its rated current value with generator speeds of rotation of between 4000 and 6000 rpm.
Modern motor vehicles have rated generator currents of between 65 and 110 amperes, rated currents of 140 amperes being already known in the case of high-performance production cars.
The present density of traffic and the constantly increasing number of current-consuming devices in cars necessarily leads to larger battery capacities and to electrical generators which are capable of greater power, i.e. are larger and require more drive energy.
"Stop and go" traffic can, for instance, have the result that while the car can be driven to its parking place with its own drive power, but because there has been no charging of the battery but only a discharge, the current supplied by the battery is no longer sufficient to start the internal combustion engine again by means of the electric starter.
In order that charge current of the generator is available even at idling speed of the internal combustion engine (about 700 rpm), transmission ratios of 2:1 have been provided, so that the generator rotates with twice the speed of rotation of the engine. In order further to increase the charge current upon idling speed of rotation, even higher transmission ratios would be required. With the highest speed of rotation of the engine, however, the speed of rotation of the generator attains speeds which cannot be controlled by traditional, economically feasible means. For this reason, it has been attempted to avoid these difficulties by means of a switchable transmission, but the expense and the considerably large structural dimensions have prevented the introduction thereof in vehicles.
Solutions which increase the voltage of the electrical system to 24 volts in order in this way to have fewer electrical losses due to decreased current requirements in the case of a large number of devices have also been discussed Such an increase in voltage, however, also results in a large number of disadvantages such as, for instance, a thinner incandescent filament in incandescent lamp bulbs, which makes focusing difficult and shortens the lifespan and larger transient voltages due to inductive components, as well as the danger of the occurrence of arcs at contacts, and the need to transform the voltage for standard digital and analog components which operate today at the 12 volt level, etc.